Towing the line

Fish receive much of their sensory information — used, for example, to guide their stunning schooling behaviour — from a particular sensory organ system, the lateral line. Hair cells sitting in neuromast organs that are scattered along the flank of the fish sense water movements, much as hair cells in our inner ear sense air movements. The lateral line provides the fish with information about movements of prey or of itself, as well with a sense of its environment. As it turns out, by looking at the development of the lateral line, biologists are beginning to learn how complex organ systems are built. In the embryo of the zebrafish, the lateral line is built from three components: the neuromasts, which provide sensory input; a nerve that connects the neuromasts to the brain; and glia cells that wrap around the nerve. The individual neuromasts are dropped off by a heap of precursor cells, the lateral line primordium, which migrates from head to tail along the flanks of the zebrafish embryo. Likewise, the end of the nerve grows from front to back, along the same path as the primordium. Now, as published in Nature Neuroscience (7:491–492), Darren Gilmour and colleagues in Tübingen have addressed the question of who guides whom in the forming lateral line. Does the nerve follow the primordium or vice versa or are they guided independently? Gilmour and colleagues have shown that the growing nerve always stays with the migrating primordium. If the primordium is rendered unable to receive a guidance signal, it is misled and will migrate to the wrong places; but wherever it goes, the nerve’s growing end will always follow, even if it is able to receive the signal itself. This means that the primordium is the guide and provides the cue, which the nerve will follow, a phenomenon referred to as “towing”. Essentially, towing assures that a nerve ending becomes associated with every neuromast the primordium drops off. That a growing nerve has to follow a moving target while it is migrating itself, contrasts with the formation of neural connections in other systems, in which nerve endings are guided through a static environment. Insight into the lateral line system might therefore also help throw light on coordinated cell migration during organ formation in general. It will be fascinating to see what more the study of the lateral line system has in tow. Towing the line